It has been hitherto known that properties such as heat insulation, sound absorption, buoyancy, elasticity and weight reduction are improved by mechanically foaming plastics, foaming by using reactive gas or foaming by using foaming agents, and such foamed plastics are widely used in the soundproof material, heat insulator, building material, lightweight structural material, packaging material, insulating material, cushion, material, quakeproof material, footwear, automobile interior, and others. Many usable resins are known, and various foaming techniques are also known.
Recently, in order to save energy, save resources and save space, it is desired to improve the heat insulation and soundproof performance with a material that is thin as possible.
To improve the heat insulation or soundproof performance per unit thickness, it is desirable to maintain a vacuum inside of the foamed plastics. In using a vacuum pump as generally known, it is impossible to maintain a vacuum individually in the foamed plastics, and the heat insulation and soundproof qualities are not satisfactory.